Try test suite against macOS and Windows

[simonw]

Refs #1625

[simonw]

That broke on a macOS test: https://github.com/simonw/datasette/runs/5044036993?check_suite_focus=true

I'm going to remove macOS and Ubuntu and just try Windows purely to see what happens there.

[codecov]

Codecov Report

Merging #1626 (4b4d0e1) into main (b5e6b1a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1626   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files          34       34           
  Lines        4531     4531           
=======================================
  Hits         4176     4176           
  Misses        355      355           

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[simonw]

OK, the tests do NOT pass against Windows! https://github.com/simonw/datasette/runs/5044105941